An Edwardian ring from the start of the Art Deco period.
Lovely piece of great charm and elegance.
Three old mine cut diamonds . Especially clear and sparkly for the period. 
It has not been sized but because of the design will size easily. This is fortunate as not many of us these days are a size I (UK).
4 1/4 ( US). Those ladies must have been so dainty. Dates around 1905 -1920.
The setting for the diamonds tests as  platinum. The shank tests as 18ct gold.
The three diamonds are that lovely squarish shape of the old mine cut. 
The central diamond measures 4mm by 4mm and is very deep - getting on for 0.5 carat.
The two side diamonds are 2mm by 2mm. All matched in colour and sparkle.